Portable radiation measurement instruments, commonly referred to as “survey” instruments, are available commercially. Such instruments may also be referred to as “radiac” instruments, where “radiac” is an acronym for radioactive detection, identification, and computation. The term refers to the detection and measurement of the intensity of emitted nuclear radiation. Some of these survey or radiac instruments comprise both neutron and gamma ray detectors thereby responding to both impinging neutron and gamma radiation. Portable survey instruments vary in size from small, pager-size devices that fit on a person's belt to larger instruments that are the size of a suitcase or larger. Capabilities and response characteristics of these survey instruments vary considerably. Some of the larger survey instruments contain sophisticated radiation spectrometer detectors with isotope identification and analysis capability, and are relatively sensitive to impinging radiation. Pager-size survey instruments typically contain small radiation detectors and are relatively insensitive to impinging radiation and lack sophisticated analysis capability. There are intermediate size survey instruments with intermediate radiation sensitivity and intermediate analysis capability. Directional radiation response capabilities are typically obtained with radiation collimators, which add significantly to the size and weight of the survey instrument.
Emphasis on security and antiterrorist activities has renewed interest and intensified the need for radiation survey instrumentation. As an example, radiation survey meters are used to monitor incoming cargo for clandestine radioactive materials or weapons containing radioactive materials. These applications require instrumentation and data processing methodology that can be used to determine the type (e.g. gamma radiation, neutron radiation, or both) and the intensity of detected radiation. In these applications, it is also highly desirable to have a survey instrument with directional capabilities. This allows the user of the instrument to more easily determine the physical location of a source of radiation. Using the above example, a directional survey instrument can assist in locating clandestine radioactive material within a cargo container containing additional, non-radioactive material.